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The forest as an indicator of organic pollutants

Jérôme Poulemard (Savoie Mont Blanc University) takes the floor

The forest as an indicator of persistent organic pollutants in the atmosphere accumulating in the environment: the example of polycyclic aromatic hydrocarbons (PAHs)

Summary

Polycyclic aromatic hydrocarbons (PAHs) are a family of persistent organic pollutants (POPs) originating essentially from the incomplete combustion of organic matter. The European Union has singled out PAHs for priority action due to their toxicity and generalised presence throughout the environment (they are often dispersed through the atmosphere and deposited far from their point source).

Within the framework of a doctoral thesis, PAH concentrations in tree leaves and in forest humus and soil were studied based on samples preserved in the soil collection taken from 1993 to 2011 on 14 RENECOFOR plots.

High PAH levels were occasionally found at certain sites near local emission point sources (forest fire sites, nearby industrial or urban areas). When these particular situations were excluded, however, PAH concentrations in the leaves and needles (sampled each year) regularly decreased throughout the study period (see figure below).

Evolution of the concentrations sum of the PAHs analyzed in leaf samples from 14 plots of the RENECOFOR network
Evolution of the concentrations sum of the PAHs analyzed in leaf samples from 14 plots of the RENECOFOR network © Sara Negro / Université Savoie Mont Blanc

This decrease was concomitant with a decrease in the atmospheric PAH emissions estimated for the same period. This shows that analysing historic leaf samples can reveal changing trends in atmospheric HAP levels, at least to a certain degree. Based on measured concentration levels, an estimated 2 to 5% of global PAH emissions were fixed in the leaves of French forests. These results are far below the 44±18% initially proposed by Simonich and Hites (1994) in Indiana (USA), and indicate that forests may have less of a filtering effect on atmospheric pollutants than is often suggested.

In the humus, PAH concentrations seem to be mostly dependent on processes related to mineralisation and the incorporation of organic matter into the soil. Indeed, higher concentrations were (found in the OH horizon of the thickest humus types (mor-humus formation). Nevertheless, the lightest PAHs were likely mobilised via the water flux.

Below the humus, very high PAH stocks were found in the soil mineral layers compared to incoming fluxes, thus indicating a very long residence time. The build-up of PAHs in French forest soils could reflect a very long period of accumulation (several hundred years).

In this study, the RENECOFOR Network proved to be extremely pertinent for retro-active environmental observations thanks not only to its network of permanent plots, but also to its sample collections (soil collection, leaf archives).

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